Mechanical seals between a rotatable shaft and its stationary supporting housing, may include annular sealing rings respectively sealed and connected or keyed relative to the shaft and to the housing, and having annular sealing faces disposed facing one another. The relative rotation of the shaft and housing takes place between these sealing faces; and they are adapted to cooperate closely with one another to minimize fluid leakage between the faces, from a high pressure at one radial edge of the sealing faces to a lower or atmospheric pressure at the opposite radial edge.
At least one of the sealing rings generally is movable axially of the shaft, and biased by springs or the like, to urge the annular sealing faces closely proximate, or even substantially against one another, particularly when the shaft is not being rotated. However, to reduce wear between the sealing faces, as they are being moved relative to one another when the shaft is being rotated, it is intended that a small fluid film gap be established between the sealing faces. The gap thickness is determined when the forces acting on the moving ring member balance; such forces include the hydrostatic and hydrodynamic fluid pressure acting on the opposite faces of the member, and the force of the closing spring. The fluid forces may result from the contained fluid, although a secondary lubricating fluid could also be used.
When properly designed, the fluid film gap between the sealing faces may be measured in tenths-of-thousandths of an inch (0.0001"), and the leakage through the gap is minimal. This sealing gap nonetheless eliminates or minimizes direct contact between the sealing faces, to significantly reduce wear of such faces; and further to significantly reduce frictional drag, heat buildup and power consumption of the mechanical seal. Adjacent sealing faces require very close tolerances, and, during operation, the adjacent sealing faces must remain true and parallel relative to one another.
Precise positioning of the seal rings with respect to each other is particularly critical in dry running gas seals. Only the contained fluid itself, typically a gas, is used. Gas generally is not an effective lubricant and improved control of hydrostatic and hydrodynamic forces is relied on to provide a fluid film gap between the sealing faces, so as to avoid direct surface contact of the sealing faces. Even so, the gap between the sealing faces must be very small to contain a gas, without allowing excessive leakage.
A successful design of a dry running seal has a plurality of circumferentially spaced shallow radial grooves provided in one of the sealing faces. These grooves are spirally disposed from the edge of the sealing face that is adjacent the high pressure contained fluid and extend part way across the face toward the edge adjacent the low pressure. The pressure of the contained fluid at the high pressure edge is presented via the grooves to an interior region of the sealing faces, to provide for a hydrostatic pressure tending toward separation of the sealing faces. As the sealing faces rotate relative to each other, the pumping action of the grooves creates a hydrodynamic force that separates the faces to some gap, until the forces are counterbalanced by the forces of the springs and of the opposing fluid forces tending to close the gap. This gap avoids direct contact between the sealing faces. U.S. Pat. Nos. 3,499,653, 4,212,475 and 4,768,790 disclose specific embodiments of mechanical dry running gas seals. All three patents are commonly assigned to the assignee of the present invention and the teachings of these patent, where appropriate, are incorporated herein by reference.
In a mechanical seal of the type disclosed in the above mentioned patents, the sealing and/or film leakage gap is dynamic and changing. When the shaft is not rotating the seal faces are in contact. As the shaft rotates, the pumping action of the seal face grooves causes the faces to move apart thus creating a gap. Other factors such as axial shaft movement, thermal distortion of parts, pressure variations, etc. require that one seal face accommodate axial movement. One sealing ring is, therefore, movable axially along the shaft. As noted, the gap for a dry running seal may be measured in tenths-of-thousandths of an inch (0.0001"), and the movement of the one sealing ring is comparable. Any resistance hindering the movable sealing ring in this very small range detracts from the sensitivity of the mechanical seal. Excessive resistance preventing compensating movement of the movable ring may hold the gap between the sealing surfaces too small, causing wear, overheating and/or even destruction of the adjacent sealing faces; or may keep an open gap from closing, allowing excessive leakage.
Of importance also is the fact that forces on the sealing faces must be reasonably uniform and symmetrical with respect to the sealing faces and to the rotational axis of the sealing faces. This means that the sealing faces must be properly centered relative to one another. If the sealing faces are offset, from a coaxial position, or caused to move out of parallel relative to one another, the sealing faces at one local region may be forced too close together or even to contact one another, while they may be spaced apart excessively at another spaced local region. When the sealing faces are offset, both wear and leakage problems are aggravated. These factors are augmented by any non-coaxial position of the seal rings.
Centering of the relatively rotating sealing rings is important to seal performance and to seal component life. If one ring is disposed in a position offset from the axial centerline of the other, unequal loading can occur, for example, from the fluid pumped between the sealing faces by the spiral pumping grooves.
In prior designs having seals exposed to the high pressure to be sealed at the outside diameter of the seal rings, it has been adequate to center the seal rings at the inside diameter. For operating pressures and temperatures previously experienced, such an approach had been adequate. However, as pressure and temperatures experienced in seal environments have become more severe, an improved means of centering of the seal rings becomes important.
An important consideration in mechanical face seals is the requirement that each ring have a retaining means which prevents its rotation relative to the structure with which that ring is associated. For example, the stationary ring must not rotate with respect to the housing. Similarly, the rotating ring must remain stable with respect to the rotating shaft so that the ring and shaft rotate together.
To insure that each ring is retained in a fixed, nonrotatable position relative to the housing and shaft, respectively, various techniques have been utilized in the prior art. One technique, for example, is a retainer supporting one of the rings with pins extending from the retainer and into the retained ring. The pins engage appropriate keyways either at the edges or in the body of the ring. Pins present a difficulty, however, in that they are assembled as separate parts from the retainer and are apt to become lost or misaligned with respect to the ring and retainer during assembly or disassembly.
One approach to overcoming these difficulties has been to provide plural convex ridges or protuberances on the inside face of the retainer. The ridges then slip into keyway slots machined into the edges of the ring. The ridges provide firm, non-rotational retention of the ring by the retainer through a tight frictional fit in the keyway slots., The retainer itself is directly or indirectly attached to either the housing or the shaft, depending on which of the rings is retained.